1. Field of the Invention
The present invention relates to a spherical form measuring and analyzing method using an interferometer apparatus; and, more specifically, to a spherical form measuring and analyzing method which determines overall form information of a surface to be inspected brighter than a reference lens for spherical surface measurement by combining respective form information items of individual partial regions of the surface to be inspected.
2. Description of the Prior Art
A reference lens is used when measuring the overall form of a surface to be inspected such as a spherical lens or a reflecting mirror by an interferometer apparatus. When the surface to be inspected is brighter (specifically represented by a greater numerical aperture, a smaller F value, or a greater aperture size) than the reference lens, it is theoretically impossible to measure the whole region of the surface at once. Therefore, a technique known as aperture synthesis has been in use.
In general, aperture synthesis has been known as a technique in which the whole region of an object having a large area is divided into partial regions, each partial region is measured while adjacent partial regions overlap each other, and thus measured respective form information items of the individual partial regions are put together according to the form information concerning the regions measured in an overlapping manner, so as to determine form information of the whole region of the object.
In measurement and analysis using an interferometer apparatus, such aperture synthesis processing is relatively easy to apply to a flat object, but hard to apply to a spherical object due to its spherical surface.
When measuring a spherical surface to be inspected which is brighter than a reference lens by using an interferometer apparatus, a regional interference fringe image corresponding to a partial region of the spherical surface is captured each time the spherical surface is relatively moved with respect to the reference lens by a predetermined amount. In each regional interference fringe image, information of a three-dimensional space, i.e., a spherical surface to be inspected having a predetermined curvature, is transformed into information of a two-dimensional space, i.e., an imaging plane of CCD or the like. Each regional interference fringe image is also an image whose center position corresponds to the center position of the imaging plane. Therefore, even when parts of the respective regional form information items obtained from the regional interference fringe images are known to belong to the same region, these regional form information items cannot simply be put together in a plane coordinate system of the imaging plane.
Though literatures Japanese Unexamined Patent Publication Nos. HEI 2-259509 and HEI 10-281737, etc.) disclosing such aperture synthesis of spherical surfaces have conventionally been known, none of them has specifically described how the respective regional form information items obtained from captured regional interference fringe images can be put together.